Method for treating amonia-containing effluent water

ABSTRACT

Disclosed are a method and an apparatus for treating an effluent containing ammonia in which method and apparatus N 2 O concentration in the gas at the outlet of a catalyst tower does not rise to a high level even when the NH 3  concentration in the effluent was reduced and the amount of hazardous substances formed is small; in the method and apparatus, an NH 3 -containing effluent A and a carrier gas (steam C and combustion gas F) are contacted in stripping tower  7  to transfer the NH 3  from the NH 3 -containing effluent to a gas phase, the gas containing the generated NH 3  is heated with pre-heater  19  and then contacted with catalyst layer  13  placed in catalyst tower  12  to decompose the NH 3  into nitrogen and water; and at that time, the oxygen concentration in the gas to be introduced into catalyst tower  12  and the N 2 O concentration in the gas discharged from catalyst tower  12  are determined by measuring instruments  21  and  22 , respectively, and the oxygen concentration in the gas to be introduced into catalyst tower  12  is adjusted by adjusting valve  17  so that the N 2 O concentration becomes within a prescribed range.

TECHNICAL FIELD

The present invention relates to a method for treating an effluent(waste water) containing ammonia(NH₃). More specifically, the presentinvention relates to a method and an apparatus for treating anNH₃-containing effluent by which method or apparatus the ammoniacontained in the effluent discharged especially from a thermal powerplant is efficiently converted into nitrogen (N₂) and water (H₂O) tomake the ammonia harmless by a stripping method.

BACKGROUND ART

In recent years, there has been a growing concern to the conservation ofglobal environment, and regulations against over-fertilization of seaareas have been enforced. Thus, the development of a new technology forremoving nitrogen from an effluent has been sought. In answer to suchrequest, the removal of the nitrogen contained in an effluent has beenconducted from some time ago mainly by the following methods:

-   1) Biological denitrification method: Method in which an organic    nitrogen contained in water is converted into an inorganic nitrogen    to render the organic nitrogen harmless by using a bacterium.-   2) Discontinuous NH₃ decomposition method with chlorine: Method in    which NH₃ is oxidized to decompose by using sodium hypochlorite.-   3) Ion exchange method: Method in which NH₃ is adsorbed on a zeolite    through an ion exchange.-   4) Ammonia stripping method: Method in which NH₃ is diffused or    evaporated from an NH₃-containing effluent into the atmosphere by    using air or steam.

When the BOD (biochemical oxygen demand) of an effluent is high,biological denitrification method 1) described above is used. On theother hand, in the case where an effluent in which most of nitrogen isin a form of ammonia nitrogen such as ammonia and ammonium ion is to betreated, for instance, when an effluent from a process in a chemicalfactory or an effluent once-subjected to a post-treatment is the objectof the treatment, method 2), 3) or 4) is used.

However, the conventional methods described above have the problems asfollows:

In the method 1), the size of a reaction bath necessary for thetreatment becomes large since the rate of a biological reaction is slow,and thus a large space becomes necessary for placing the reaction bath.Besides, the method 1) raises the problem that excess amount of a sludgeis produced. Method 2) causes the problem that a treatment of remainingchlorine becomes necessary and organic chlorine compounds are formed,since the addition of sodium hypochlorite in an amount more than thatstoichio-metrically required is necessary for completely removing theammonia. In the method 3), a secondary effluent containing ammonium ionat a high concentration is produced at the time of regenerating a usedzeolite and thus a treatment of the secondary effluent becomesnecessary. Further, the method 4) has the problem that an NH₃-containinggas is diffused or dissipated into the atmosphere after the NH₃ wastransferred into a gas phase and causes a secondary pollution.

Among the methods described above, method 4) is advantageous comparedwith other methods since the treatment of an effluent is comparativelysimple and the costs of equipments and operations are small.Accordingly, a combination in which the method 4) is performed incombination with another method which can be used for oxidizing todecompose the NH₃ contained at a high concentration in a gas separatedfrom an effluent, by using a catalyst, to make the NH₃ contained in theeffluent harmless as the result of the combination has been adopted evenin current night-soil treatment facilities. However, in such a strippingand catalytically oxidizing process, it is necessary to install acatalyst tower for reducing NOx in addition to a catalyst tower foroxidizing NH₃ since a large quantity of NOx is generated at the time ofthe oxidation of NH₃. Further, according to the investigations by thepresent inventors, it has been found out anew that a large quantity ofN₂O is also produced in this process at the time of oxidizing the NH₃.Like CO₂, N₂O is a substance contributing to the global warming.Accordingly, it is dangerous to the global environment that a largequantity of N₂O is diffused into the atmosphere, in the same extent asNH₃ is diffused as it is. Thus, the diffusion of N₂O is alsoundesirable.

As described above, treatments of NH₃-containing effluents inconventional technology have many problems and some of the treatmentshad a problem that they might become sources from which varioussecondary pollution substances are produced anew.

DISCLOSURE OF THE INVENTION

Subject of the present invention is to propose a method and an apparatusfor treating an NH₃-containing effluent in which method and apparatusthe amount of secondary pollution substances formed is reduced and theamount of utilities such as steam to be used can also be reduced.

In order to achieve the subject described above, the method andapparatus of the present invention are summarized as follows:

-   (1) A method for treating an ammonia-containing effluent comprising    a stripping step in which the ammonia (NH₃) contained in the    NH₃-containing effluent is transferred with a carrier gas from the    effluent into a gas phase, a step for adding an oxygen-containing    gas to the NH₃-containing gas produced at the stripping step, and an    NH₃ decomposing step in which the oxygen-containing gas and the    NH₃-containing gas are contacted with one or more kind of catalysts    used for decomposing NH₃, at a prescribed temperature to decompose    the NH₃ into nitrogen and water, the concentration of oxygen in the    gas mixture introduced into the NH₃ decomposing step being adjusted.-   (2) The method for treating an NH₃-containing effluent recited in    paragraph (1) above wherein the method further comprises a step by    which a part of the gas resulted in the NH₃ decomposing step is    discharged outside the effluent treating system and the remaining    part of the gas resulted in the decomposing step is recycled as a    part of the carrier gas to be used in the stripping step.-   (3) The method for treating an NH₃-containing effluent recited in    paragraph (1) or (2) above wherein the concentration of oxygen in    the gas mixture to be introduced into the NH₃ decomposing step is    adjusted to a value within the range of 2 to 15%.-   (4) The method for treating an NH₃-containing effluent recited in    any one of paragraphs (1) to (3) above wherein the concentration of    oxygen in the gas mixture to be introduced into the NH₃ decomposing    step is adjusted so that the concentration of the N₂O in the gas    resulted in the NH₃ decomposing step becomes a value within a    prescribed range.-   (5) The method for treating an NH₃-containing effluent recited in    any one of paragraphs (1) to (4) above wherein the catalyst used for    decomposing NH₃ comprises a first component having an activity of    reducing nitrogen oxides with NH₃ and a second component having an    activity of forming nitrogen oxides (NOx) from NH₃.-   (6) The method for treating an NH₃-containing effluent recited in    any one of paragraphs (1) to (5) above wherein the catalyst used for    decomposing NH₃ comprises, as a first component, an oxide of    titanium (Ti) and an oxide of one or more elements selected from the    group consisting of tungsten (W), vanadium (V), and molybdenum (Mo),    and, as a second component, a silica, zeolite, and/or alumina having    one or more noble metals selected from the group consisting of    platinum (Pt), iridium (Ir), rhodium (Rh), and palladium (Pd)    supported thereon.-   (7) The method for treating an NH₃-containing effluent recited in    any one of paragraphs (1) to (5) wherein the catalyst used for    decomposing NH₃ is a zeolite or comprises, as a main component, a    zeolite.-   (8) The method for treating an NH₃-containing effluent recited in    any one of paragraphs (1) to (4) above wherein the concentration of    oxygen in the gas mixture to be introduced into the NH₃ decomposing    step is adjusted so that the concentration of the NH₃ remaining in    the gas resulted in the NH₃ decomposing step becomes a value within    a prescribed range, while using the concentration of the NH₃    remaining in the gas resulted in the NH₃ decomposing step as an    index, instead of the concentration of the N₂O in the gas.-   (9) The method for treating an NH₃-containing effluent recited in    paragraph (8) above wherein the concentration of the NH₃ remaining    in the gas resulted in the NH₃ decomposing step is higher than 50    ppm.-   (10) The method for treating an NH₃-containing effluent recited in    paragraph (8) or (9) above wherein the gas pressure in the effluent    treating system is controlled to a prescribed value so that the    amount of a part of the gas resulted in the NH₃ decomposing step and    discharged outside the system becomes equal to the increase of the    total amount of the gas including the amount of the    oxygen-containing gas supplied into the system.-   (11) The method for treating an NH₃-containing effluent recited in    any one of paragraphs (8) to (10) above wherein the method further    comprises a step for removing ammonia from a part of the gas    resulted in the NH₃ decomposing step after the part of the gas was    discharged outside the system.-   (12) The method for treating an NH₃-containing effluent recited in    any one of paragraphs (1) to (4) above wherein the step for    decomposing the NH₃ into nitrogen and water by contacting the    NH₃-containing gas with a catalyst is

a step in which two or more kind of catalysts each having a differentpower for oxidizing NH₃ are used, and the NH₃-containing gas iscontacted first with a catalyst having a relatively low power foroxidizing NH₃ to decompose a part of the NH₃ into nitrogen and water andthen with a catalyst having a relatively high power for oxidizing NH₃ todecompose the remaining part of the NH₃ into nitrogen and water, or

a step in which the NH₃-containing gas is contacted at the same timewith two or more kind of the catalysts to decompose the NH₃ intonitrogen and water.

-   (13) The method for treating an NH₃-containing effluent recited in    paragraph (12) above wherein two or more kind of the catalysts each    having a different power for oxidizing NH₃ comprise, as a first    component, an oxide of titanium (Ti) and an oxide of one or more    elements selected from the group consisting of tungsten (W),    vanadium (V), and molybdenum (Mo), and, as a second component, a    silica, zeolite, and/or alumina having one or more noble metals    selected from the group consisting of platinum (Pt), iridium (Ir),    rhodium (Rh), and palladium (Pd) supported thereon, and the power    for oxidizing NH₃ is adjusted by the ratio of the content of the    first component to that of the second component.-   (14) The method for treating an NH₃-containing effluent recited in    paragraph (12) above wherein the catalyst having a relatively high    power for oxidizing NH₃ is a zeolite.-   (15) An apparatus for treating an NH₃-containing effluent comprising    a stripping means for transferring the ammonia (NH₃) contained in    the NH₃-containing effluent with a carrier gas from the effluent    into a gas phase, a means for adding an oxygen-containing gas to the    NH₃-containing gas produced in the stripping means, an NH₃    decomposing means by which the oxygen-containing gas and the    NH₃-containing gas are contacted with one or more kind of catalysts    used for decomposing NH₃, at a prescribed temperature to decompose    the NH₃ into nitrogen and water, and a means for adjusting the    concentration of oxygen in the gas mixture to be introduced into the    NH₃ decomposing means.-   (16) The apparatus for treating an NH₃-containing effluent recited    in paragraph (15) above wherein the apparatus further comprises a    means for determining the concentration of N₂O or NH₃ in the gas    discharged from the NH₃ decomposing means and controlling the    concentration to a value within a prescribed range.-   (17) The apparatus for treating an NH₃-containing effluent recited    in paragraph (15) or (16) above wherein the catalyst used for    decomposing NH₃ comprises, as a first component, an oxide of    titanium (Ti) and an oxide of one or more elements selected from the    group consisting of tungsten (W), vanadium (V), and molybdenum (Mo),    and, as a second component, a silica, zeolite, and/or alumina having    one or more noble metals selected from the group consisting of    platinum (Pt), iridium (Ir), rhodium (Rh), and palladium (Pd)    supported thereon.-   (18) The apparatus for treating an NH₃-containing effluent recited    in paragraph (15) or (16) above wherein the catalyst used for    decomposing NH₃ is a zeolite or comprises, as a main component, a    zeolite.-   (19) The apparatus for treating an NH₃-containing effluent recited    in paragraph (15) above wherein the catalyst used for decomposing    NH₃ comprises

a catalyst in which one or more catalyst layers having a relatively lowpower for oxidizing NH₃ and one or more catalyst layers having arelatively high power for oxidizing NH₃ are arranged in series, or

a plate-like catalyst in which catalyst layers having a relatively lowpower for oxidizing NH₃ and catalyst layers having a relatively highpower for oxidizing NH₃ are arranged alternately in the directionperpendicular to the direction of the gas flow with the surfaces of thelayers being held in parallel to the gas flow direction.

As specific examples of catalysts comprising a first component having anactivity of reducing nitrogen oxides with NH₃ and a second componenthaving an activity of forming nitrogen oxides (NOx) from NH₃, catalystscomprising, as a first component, an oxide of titanium (Ti) and an oxideof one or more elements selected from the group consisting of tungsten(W), vanadium (V), and molybdenum (Mo), and, as a second component, asilica, zeolite, and/or alumina having one or more noble metals selectedfrom the group consisting of platinum (Pt), iridium (Ir), rhodium (Rh),and palladium (Pd) supported thereon can be mentioned. Besides,catalysts substantially consisting of zeolite or comprising, as a maincomponent, zeolite have an effect of considerably reducing the formationof NOx or N₂O.

By changing the ratio of the first component to the second component inthe catalyst described above, it is possible to adjust theconcentrations of NH₃ and NOx in the gas resulted in the NH₃decomposition. For instance, when the ratio of the second component wasreduced, the ratio of decomposition of NH₃ is slightly lowered, but theconcentration of NOx in the resulting gas is considerably reduced.

In order to transfer the NH₃ contained in an NH₃-containing effluentfrom the effluent into a gas phase, a method in which the NH₃ containedin the effluent is stripped into the gas phase, specifically, forexample, (a) a method in which a carrier gas is blown into the effluent,and (b) a method in which the effluent is sprayed in a carrier gas areused. When the effluent has a pH of 10 or higher, the stripping isperformed as it is. On the other hand, when the effluent has a pH oflower than 10, an alkali such as sodium hydroxide and calcium hydroxide(slaked lime) is first added to the effluent to make its pH 10 orhigher, and then the effluent is contacted with air to diffuse orevaporate the NH₃ into the air by using the air as a carrier gas. As thecarrier gas, steam can be used in place of air. The term “carrier gas”as used herein generically means a gas which gas can diffuse orevaporate ammonia from the effluent.

An NH₃-containing gas is preheated at the time when the gas isintroduced into a stripping tower or catalyst tower, when necessary.Preheating may be conducted by a usual method, for example, by heatingwith a burner or heat exchange with a gas at a high temperature such assteam or a gas discharged from a catalyst device. When the gas iscirculated in the method and apparatus of the present invention, it ispreferable to use a method in which the composition of the gas,especially the concentration of oxygen in the gas is not changed. (As anexample, a method using an indirect heat exchange is preferable.)

In the case where an NH₃ decomposing catalyst having a denitratingfunction is used, it is important to control the temperature of acatalyst layer in a catalyst tower in the range of 250 to 450° C.,preferably in the range of 350 to 400° C. In the case where a zeolitetype catalyst is used, it is preferable to maintain the temperature of acatalyst layer in the range of 450 to 600° C. In any case, it issatisfactory that a suitable temperature is selected based on theperformances of a catalyst.

The term “NH₃-containing effluent” used herein means an effluentcontaining ammonia nitrogen, such as an effluent discharged from a draintreating plant or sewerage treating facility, and an effluent dischargedfrom a dry type electrostatic precipitator or wet type desulfurizingapparatus installed for removing respectively combustion ashes or SO₂gas each contained in an exhaust gas discharged from a thermal powerplant having a coal firing boiler or oil firing boiler. Also, the term“NH₃-containing effluent” includes effluents which contain an nitrogenconverted into ammonia nitrogen by a pretreatment, such as an effluentin which an organic nitrogen originally contained in the effluent wasdecomposed into strippable ammonia nitrogen by a general biologicaltreatment, and an effluent containing NH₃ at a high concentration anddischarged at the time of the regeneration of a zeolite in aconventional ion exchange method used in various fields of industry.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram for illustrating an embodiment of the methodsfor treating an NH₃-containing effluent and the arrangements of devicesin the apparatuses of the present invention.

FIG. 2 is a line graph showing the relation between the concentration ofoxygen gas introduced into a catalyst tower, and ammonia decompositionratio and N₂O concentration in the resulting gas when anammonia-containing effluent was treated in the embodiment shown in FIG.1.

FIG. 3 is a flow diagram similar to that of FIG. 1, showing anotherembodiment of the present invention.

FIG. 4 is a flow diagram similar to that of FIG. 1, showing stillanother embodiment of the present invention.

FIG. 5 is a line diagram showing the relation between the concentrationof ammonia and the concentration of the sum of NOx and N₂O in the gasdischarged from a catalyst tower in the embodiment of the presentinvention shown in FIG. 4.

FIG. 6 is a flow diagram showing the method and the apparatus of thepresent invention in the case where two catalyst towers are used.

FIG. 7 is a diagram for illustrating the structure of an NH₃ decomposingcatalyst having a denitrating function and used in the presentinvention, and briefly illustrating reactions performed therein.

FIG. 8 is a diagram for illustrating the effects by which ammonia isremoved when a catalyst having a relatively low power for oxidizing NH₃and/or a catalyst having a relatively high-power for oxidizing NH₃ isused.

FIG. 9 is a diagram for illustrating the structure of a catalyst towerwhen two of the catalysts shown in FIG. 8 are used in the tower.

FIG. 10 is a line graph showing the relation between the reactiontemperature and the concentration of the sum of NOx and N₂O contained inthe gas discharged from a catalyst tower when two of the catalysts shownin FIG. 8 were used.

FIG. 11 is a diagram of a catalyst device prepared by using two kinds ofplate-like catalysts each comprising one type of catalyst, in the casewhere two catalysts shown in FIG. 8 were used.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, the embodiments of the present invention are described in moredetail with reference to drawings.

The model of a fine pore in an NH₃ decomposing catalyst having adenitrating function and used in the present invention is shown in FIG.7.

As demonstrated in FIG. 7, the fine pore has a structure in whichmicro-pores inherently contained in a porous silica exist at placeswithin a macro-pore formed by a component (first component) on thesurface of which NO is reduced by NH₃, and ultramicro-particles ofanother component (second component) having an activity of forming NOxfrom NH₃ are supported on the surface of the micro-pores of the silica.NH₃ diffuses within the macro-pore in a catalyst, the diffused NH₃ isoxidized on the second component to form NO according to the equation(1) described below, the NO collide with NH₃ adsorbed on the surface ofthe first component forming the macro-pore, in the course of diffusingoutside the catalyst, and the NH₃ is reduced down to N₂ according to theequation (2) described below. As a whole, the NH₃ is changed as shown byequation (3) described below.NH₃+5/4O₂→NO+3/2H₂O  (1)NH₃+NO+1/4O₂→N₂+3/2H₂O  (2)NH₃+3/4O_(2→)1/2N₂+3/2H₂O  (3)

As described above, it is possible to reduce NH₃ to N₂ while scarcelyforming, as a final product, NO or N₂O which is generally considered tobe formed during the process of forming NO, when an NH₃ decomposingcatalyst having a denitrating function is used, since the oxidizingreaction of NH₃ and the reducing reaction of formed NO with NH₃ proceedwithin the catalyst.

Besides, even when a zeolite is used, the amount of NO or N₂O formed isextremely small.

However, even in the case where such catalyst is used, a phenomenon inwhich the concentration of N₂O at the outlet of a catalyst tower becomesslightly high when the concentration of NH₃ in an effluent was high wasobserved. As a result of diligent investigations by the presentinventors, it has been found out that the means described below iseffective to such increase of the concentration of N₂O at the outlet ofa catalyst tower.

When the concentration of oxygen in a gas within a catalyst tower(herein, the concentration of oxygen in a gas to be introduced into acatalyst tower) is low, ammonia decomposition ratio and theconcentration of the N₂O contained in the resulting gas decrease, butthe decrease in the concentration of N₂O occurs at a higherconcentration of oxygen than the oxygen concentration where the ammoniadecomposition ratio comes to decrease. In an example, whereas the oxygenconcentration at which the ammonia decomposition ratio begun to decreasewas 3% or lower, N₂O concentration begun to decrease when the oxygenconcentration became 10% or lower. Thus, it has been found out that theN₂O concentration can be reduced without lowering ammonia decompositionratio by maintaining the oxygen concentration at a proper value (herein,2 to 15%, preferably 2 to 10%).

FIG. 1 is a flow diagram showing a system of devices in the case whereina method of the present invention for treating an NH₃-containingeffluent is applied to an effluent containing ammonia nitrogen at a highconcentration, for example, to an effluent discharged from a thermalpower plant.

As shown in FIG. 1, effluent A and alkali B are supplied to tank 3through pipe 1 and pipe 2, respectively, mixed within tank 3, and thenfed to pre-heater 5 with pump 4. The effluent A preheated up to about100° C. with pre-heater 5 is supplied to a top portion of strippingtower 7 through pipe 6. Within stripping tower 7, filler material 8 isplaced. Steam C and combustion gas F are supplied as carrier gasesthrough pipe 9 and pipe 16 connected to bottom portions of the tower,respectively, and rise within the tower while efficiently contactingwith effluent A within the tower to obtain a gas containing ammonia at ahigh concentration. The concentration of NH₃ in the gas thus obtained isusually a few thousands to a few tens of thousands ppm. To the gas thusobtained is a proper amount of air D supplied through pipe 18 byadjusting the opening of regulating valve 17 according to the signalsfrom a device for measuring oxygen concentration and a device formeasuring N₂O concentration both described below.

Another combustion gas F supplied from a combustion device (not shown inthe drawing) is fed into pre-heater 19 through pipe 20, mixed thereinwith the gas containing ammonia, heated up to a prescribed temperature,and then introduced into catalyst tower 12. Near the inlet of catalysttower 12, device 21 for measuring oxygen concentration is installed, andthe oxygen concentration in the gas is determined with the device. Thestripped ammonia contained in the gas is oxidized to decompose into N₂and H₂O on catalyst (layer) 13, and then discharged into the atmospherethrough pipe 14.

The concentration of N₂O at the outlet of the catalyst tower 12 isdetermined with device 22 installed near the outlet of catalyst tower 12and used for measuring N₂O concentration, the determined value thusobtained and the determined value obtained by device 21 for measuringoxygen concentration are inputted into control unit 30, and the controlunit 30 controls the flow rate of air D to be supplied through pipe 18with regulating valve 17 according to the determined values. From pipe15 connected to a bottom portion of stripping tower 7, effluent E fromwhich ammonia was removed is discharged.

Two combustion gases F supplied through pipe 16 and pipe 20,respectively, may be gases other than combustion gases so far as thegases have high temperatures and low oxygen concentrations. The catalystused in catalyst tower 12 comprises a first component having an activityof reducing nitrogen oxides with NH₃ and a second component having anactivity of forming nitrogen oxides (NOx) from NH₃. Further, thereaction temperature in catalyst layer 13 at the time of operation is250 to 450° C., preferably 350 to 400° C.

According to the embodiment shown in FIG. 1, it is possible to removeammonia at a high efficiency while suppressing the formation of N₂O bycontrolling the amount of oxygen in the catalyst device. Besides,according to the embodiment shown in FIG. 3, heat loss released outsidethe system is reduced since the amount of the gas discharged outside thesystem is decreased in addition to the effect described above. Thus, asmall amount of heating energy is satisfactory in the pre-heater.

FIG. 3 is a diagram showing a system of devices in another embodiment ofthe present invention. Its difference from the system used in theapparatus shown in FIG. 1 is that a part of the gas discharged fromcatalyst tower 12 is returned to stripping tower 7 with fan 25 throughpipe 24 to employ as a part of a carrier gas thereby the amount ofheating energy can be reduced.

That is, Gas G containing N₂ and H₂O formed by the decomposition of theammonia leaves catalyst tower 12 and passes through pipe 24, a part ofthe gas is discharged through pipe 23 into the atmosphere, and theremaining part of the gas is returned to stripping tower 7 with fan 25.It is the same as in the case of FIG. 1 that the concentration of N₂O atthe outlet of catalyst tower 12 is determined by N₂O concentrationmeasuring device 22 installed at a midway of pipe 24, the determinedvalue thus obtained and the determined value obtained by oxygenconcentration measuring device 21 are inputted into control unit 30, andthe control unit 30 controls the flow rate of the air supplied throughpipe 18, with regulating valve 17 according to the determined values.

Whereas steam C and combustion gas F (supplied through pipe 16) are usedas a stripping gas in the case of the apparatus shown in FIG. 3, pipe 16(for supplying combustion gas F) can be omitted, for example, bycontrolling the amount of the circulating gas from pipe 24, or byproviding a pre-heater at pipe 9.

Whereas N₂O concentration measuring device 22 is used in the system ofdevices shown in FIG. 1 and FIG. 3, it is not necessary to always usethe device 22. For instance, it is sufficient that the range of properoxygen concentrations at which ammonia decomposition ratio is high andN₂O concentration in the gas discharged from a catalyst tower can bedecreased lower than a prescribed value is ascertained in advance andthen the oxygen concentration is adjusted so as to become aconcentration within the ascertained range.

FIG. 4 shows the same diagram of the system of devices as shown in FIG.3 with the exception that N₂O concentration measuring device 22 isomitted and pipe 16 for supplying combustion gas F as a stripping gas isomitted by providing pre-heater 40.

In the apparatuses described above, air D is added through pipe 18 tothe gas discharged from stripping tower 7 and containing ammonia at ahigh concentration, and the gas mixture is introduced into catalysttower 12, after preheated up to a prescribed temperature with pre-heater19, when necessary. The amount of air D added through pipe 18 isadjusted so that the amount of oxygen contained in the air becomes equalto the amount consumed by the decomposition reaction performed oncatalyst 13. In this connection, it is possible to add oxygen gasinstead of air. The gas containing the stripped ammonia is contactedwith catalyst 13 used for decomposing NH₃ and having a denitratingfunction to oxidatively decompose the ammonia into N₂ and H₂O on thecatalyst 13 described above. The reaction temperature at this time incatalyst layer 13 is 250 to 450° C., preferably 350 to 400° C. in thecase of an NH₃ decomposing catalyst having a denitrating function, andpreferably 450 to 600° C. in the case of a zeolite type catalyst. Gas Gcontaining N₂ and H₂O formed by the decomposition of ammonia is returnedto stripping tower 7 with fan 25 as a part of a carrier gas through pipe24, after the gas temperature is raised with pre-heater 40, whennecessary. A part of gas G is discharged outside the system through pipe23 connected to pipe 24 at a position between fan 25 and pre-heater 40.It is sufficient that the amount of the gas discharged outside thesystem through pipe 23 is the same as the-amount of oxygen consumed incatalyst tower 12, specifically the same as the increased amount of agas such as air D added through pipe 18. In order to control the amountof the gas to be discharged through pipe 23, it is sufficient that thegas pressure within the system at a prescribed place is determined andthe gas is discharged so that the gas pressure at that place becomesconstant.

The moisture in gas G is condensed into water by cooling the gas with acondenser (not shown in the drawings). A slight amount of NH₃ containedin the gas may be recovered together with the condensed water.Alternatively, the NH₃ contained in the gas may be absorbed in a liquid(not shown in the drawings) containing an acid such as sulfuric acid bycontacting the discharged gas with the liquid. From pipe 15. connectedto a bottom portion of stripping tower 7, effluent E from which ammoniawas removed is discharged. While steam C supplied through pipe 9 isnecessary at the initial stage of operation, it becomes unnecessary whenthe temperature within stripping tower 7 became sufficiently high. Fan25 may be located at a place other than that shown in FIG. 4. Forinstance, the place may be the middle point between stripping tower 7and catalyst tower 12, but the location shown in FIG. 4 is preferablefrom the viewpoint of holding the inside of the catalyst tower at anegative pressure and preventing a possible gas leakage.

With respect to the composition of the gas in catalyst tower 12 shown inFIG. 4 before and after the reaction, only the amounts of NH₃ and O₂contained in the gas before the reaction decrease during the reactionand an equal amount of N₂ and H₂O are formed. In general, there is nocase where the gas composition is largely changed by the reaction sincethe NH₃ concentration in the gas to be treated is a few thousands ppm.Then, it becomes possible to circulate the gas once-subjected to an NH₃decomposition reaction to use as a part of a carrier gas, by introducingair in an amount commensurate with the amount of consumed oxygen in thesystem and taking the increased portion of the gas outside the system.By conducting such procedures, a heat source and related parts (in thiscase, one of combustion gases F and pipe 16 in FIGS. 1 and 3) forpreheating a carrier gas, and in some cases, a heat exchanger used whenthe gas discharged from a catalyst tower is preheated can be madeunnecessary.

Further, it becomes easy to remove a slight amount of NH₃ contained in adischarged gas since the amount of the gas discharged outside the systembecomes a few tenths to one hundredth of the amount in a conventionalcase. For instance, NH₃ can be recovered together with water by loweringthe temperature of the gas to condense the moisture contained in the gasinto water since NH₃ easily dissolves in water. Alternatively, it ispossible to contact the discharged gas with a liquid containing an acidsuch as sulfuric acid to have the NH₃ in the gas absorbed in the liquid.

Still further, it is possible to decrease the concentrations of NH₃ andNOx in the treated gas down to an extremely low level, since NOxconcentration reduces by using a device for measuring ammoniaconcentration in place of a device for measuring N₂O concentration usedin the system of devices shown in FIGS. 1 and 3, and conducting theoperation under the conditions wherein the NH₃ concentration in the gasat the outlet of a catalyst tower layer is increased up to a prescribedvalue.

An example of the relation between the NH₃ concentration and theconcentration of the sum of NOx and N₂O at the outlet of a catalystlayer is shown in FIG. 5. As a parameter changing the NH₃ concentrationof the abscissa, for example, the contact time between the gasdischarged from a stripping tower and a catalyst can be mentioned inaddition to the second component in the catalyst described above. Whenthe contact time between the gas discharged from a stripping tower and acatalyst was shortened by increasing the flow rate of the gas or byreducing the amount of the catalyst, the concentration of NH₃ at theoutlet of a catalyst layer increases. It is possible to make theconcentration of the sum of NOx and N₂O in the gas at the outlet of acatalyst tower lower than 1 ppm as shown by curve (a) in FIG. 5 byselecting an appropriate catalyst and maintaining the NH₃ concentrationin the gas at the outlet exit of a catalyst layer at 50 ppm or higher,preferably about 100 ppm. As such a catalyst, an ammonia decomposingcatalyst having a denitrating function can be mentioned. However, whenan appropriate catalyst is not selected, the concentration of the sum ofNOx and N₂O in the gas at the outlet of a catalyst layer can not belowered even if the NH₃ concentration in the gas at the outlet of thecatalyst layer was increased up to the highest as shown by curve (b) inFIG. 5.

Next, specific examples of the present invention are described.

EXAMPLE 1

Ammonium paratungstate ((NH₄)₁₀H₁₀.W₁₂O₄₆.6H₂O) in an amount of 2.5 kgand 2.33 kg of ammonium metavanadate were added to 67 kg of a slurry ofmetatitanic acid (TiO₂ content: 30 wt %, SO₄ content: 8 wt %) and mixedby using a kneader. The paste thus obtained was granulated, dried, andthen calcined at 550° C. for 2 hours. The granules thus obtained wereground to obtain powders as a first component of a catalyst. The powdershad a composition of Ti/W/V=91/5/4 (ratio of atoms). On the other hand,500 g of fine powders of silica (produced by Tomita Pharmaceuticals Co.,Ltd.; trade name: Micon F) was added to 1 L of 1.33×10⁻² wt % ofchloroplatinic acid (H₂[PtCl₆].6H₂O), evaporated to dryness on a sandbath, and then calcined at 500° C. for 2 hours in the air to prepare0.01 wt % Pt.SiO₂ powders as a second component of the catalyst.

Next, 5.3 kg of silica.alumina type inorganic fibers and 17 kg of waterwere added to the mixture of 20 kg of the first component and 40.1 g ofthe second component, and kneaded to obtain a catalyst paste.Separately, a net-like product made of E glass fibers was impregnatedwith a slurry containing a titania, silica sol and polyvinyl alcohol,dried at 150° C. to prepare catalyst substrates. Between the catalystsubstrates was the catalyst paste described above held and they werepassed through press rollers to roll, thereby obtaining a plate-likeproduct. After the plate-like product was air-dried in the atmospherefor 12 hours, it was calcined at 500° C. for 2 hours to obtain an NH₃decomposing catalyst having a denitrating function. In the catalyst thusobtained, the ratio of the second component to the first component (thesecond component/the first component) was 0.2/99.8.

A test for treating an effluent was conducted by using the catalystobtained and the apparatus as shown in FIG. 1 under the conditions shownin Table 1. The effects of oxygen concentration in the gas in catalysttower 12 on the decomposition ratio of ammonia and the concentration offormed N₂O in the resulting gas are shown in FIG. 2. As will be seenfrom FIG. 2, the N₂O concentration was capable to being lowered whilemaintaining a high ammonia decomposition ratio by maintaining the oxygenconcentration at the inlet of catalyst layer 13 within the range of 5 to10%.

Although it varies according to the catalyst to be used and thecomposition of the effluent to be treated, oxygen concentrationsappropriate for lowering the N₂O concentration while maintaining a highammonia decomposition ratio was 2 to 15% (more preferably 5 to 10%).

TABLE 1 Item Condition Rate of treating effluent 1.6 L/h Amount of NH₄ ⁺in effluent 2,000 mg/L Gas flow rate at inlet of 0.8 m³/h catalyst layerGas composition NH₃: 10,000 ppm H₂O: 28% Air: the remainder Temperature400° C. Areal velocity 5 m/h

EXAMPLE 2

A test for treating an effluent was conducted by using the catalystprepared by the same way as in Example 1 and the apparatus as shown inFIG. 4 under the conditions shown in Table 2. In this test, the amountof the air supplied through pipe 18 was adjusted so that the flow rateof the gas discharged outside the system through pipe 23 became 0.03m³/h. While the concentration of NH₃ in gas G discharged was 100 ppm,that of NO was 0.6 ppm, and that of N₂O was 18 ppm, the NH₃concentration was reduced down to lower than 0.1 ppm by contacting thedischarged gas with a diluted sulfuric acid. Besides, the amount ofsteam necessary for preheating a gas and liquid was kg/kg⁻.

TABLE 2 Item Condition Rate of treating effluent 1.6 L/h Amount of NH₄ ⁺in effluent 2,000 mg/L Gas flow rate at inlet of 1.3 m³/h catalyst layerGas composition NH₃: 3,000 ppm H₂O: 28% Air: the remainder Temperature350° C. Areal velocity 17 m/h

COMPARATIVE EXAMPLE 1

The first component and the second component of a catalyst were preparedby the same way as in Example 1, and then 5.3 kg of silica.alumina typeinorganic fibers and 17 kg of water were added to the mixture of 20 kgof the first component and 202 g of the second component to obtain acatalyst paste. Separately, a net-like product made of E glass fiberswas impregnated with a slurry containing a titania, silica sol, andpolyvinyl alcohol, and dried at 150° C. to prepare catalyst substrates.Between the catalyst substrates was the catalyst paste described aboveheld and they were passed through press rollers to roll, therebyobtaining a plate-like product. After the plate-like product wasair-dried in the atmosphere for 12 hours, it was calcined at 500° C. for2 hours to obtain an NH₃ decomposing catalyst having a denitratingfunction. In the catalyst thus obtained, the ratio of the secondcomponent to the first component (the second component/the firstcomponent) was 1.0/99.0.

A test for treating an effluent was conducted by using the catalyst thusobtained without circulating the treated gas under the same conditionsas in Example 1. As the result, the flow rate of the gas dischargedoutside the system through pipe 14 was 1.3 m³/h, and concentration ofthe NH₃ in the treated gas was 5 ppm, that of NO was 1 ppm, and that ofN₂O was 21 ppm. Besides, the amount of steam necessary for preheating agas and liquid was 0.25 kg/kg⁻.

From the comparison between Example 2 and Comparative Example 1, it isunderstood that in Example 2, the concentrations of NO and N₂O in thegas at the outlet of the catalyst layer were low, the gas flow ratebecame lower than 1/40, and the amount of discharged hazardous orpoisonous gases was considerably reduced compared with ComparativeExample 1. Also, the amount of steam necessary for the preheatingbecomes less than ½ of that in Example 2.

According to the embodiments shown in FIG. 4 and FIG. 6, the NH₃contained in an NH₃-containing effluent can be removed at a highefficiency at small amounts of NOx, NO, and N₂O to be formed. Further,the energy necessary for heating a liquid and a gas in the treatment ofan NH₃-containing effluent, and the amount of a gas containing ahazardous or harmful substance and discharged can considerably bereduced.

Even in the case where an NH₃ decomposing catalyst of the presentinvention having a denitrating function was used, a phenomenon in whichthe concentration of NO or N₂O in the gas at the outlet of a catalysttower became slightly high was observed when the NH₃ concentration inthe gas treated in a catalyst tower (catalyst tower 12 in FIGS. 1 to 4)was decreased to lower than a prescribed value. This is considered dueto the fact that when the reaction shown by the equation (1) describedabove was accelerated by increasing the amount of a second component ina catalyst in order to lower the NH₃ concentration in the treated gas,the concentration of NO becomes too high and the amount of NH₃ necessaryfor the reaction shown by the equation (2) described above becomesinsufficient. On the other hand, when the amount of a second componentin a catalyst was decreased, the concentrations of NOx and N₂O lower,but a problem that the concentration of NH₃ in the treated gas becomeshigh arises.

This fact is diagrammatically shown in FIG. 8. As shown in the drawing,in the case of catalyst A (case 1 in FIG. 8), the NH₃ concentration atthe end of the treatment is high since the amount of a second componentin a catalyst is small, and in the case of catalyst B (case 2 in FIG.8), the concentrations of NOx and N₂O at the end of the treatment arehigh since the amount of a second component is large. It has now beenfound out that the following methods (case 3 and case 4 in FIG. 8) areeffective for lowering simultaneously not only the concentration of NH₃but also the concentrations of NOx and N₂O in the gas at the time whenthe treatment with a catalyst was completed.

That is, an NH₃-containing gas heated up to a prescribed temperature iscontacted with a catalyst A having a relatively low power of oxidizingNH₃ to decompose a part of the NH₃ described above into nitrogen andwater. At this time, about 10% of the NH₃ contained in the original gasremains unreacted, but the concentrations of NOx and N₂O in theresulting gas are small. The NH₃ remained unchanged is then contactedwith catalyst B having a relatively high power of oxidizing NH₃ todecompose almost all remaining NH₃ into nitrogen and water (case 3 inFIG. 8). When the gas was contacted with catalyst B having a relativelyhigh power of oxidizing NH₃, NOx and N₂O tend to be formed. However, theconcentrations of NOx and N₂O in the finally resulting gas can bedecreased since the NH₃ concentration at the stage just prior to thereaction by catalyst B is lowered down to about one tenth of theoriginal concentration by catalyst A. The term “power of oxidizing NH₃”used herein means the oxidizing power per unit volume of a catalyst.

Also, a method in which an NH₃-containing gas is contacted first withcatalyst A having a relatively low power of oxidizing NH₃ to decompose apart of the NH₃ described above into nitrogen and water and thencontacted with zeolite catalyst C (case 4 in FIG. 8) is effective.Especially, this method is effective for reducing the concentration ofN₂O in the finally resulting gas since a certain type of zeolitecatalyst C has a function of decomposing N₂O.

FIG. 9 shows another example of catalyst tower 12 used in the system ofdevices shown in FIG. 1. In other words, FIG. 9 is a diagram forillustrating a catalyst device in which catalyst layer 13A having arelatively low power of oxidizing NH₃ and catalyst layer 13B having arelatively high power of oxidizing NH₃ are packed in the direction ofgas flow within the tower. In the device of FIG. 9, an NH₃-containinggas adjusted to a prescribed temperature and oxygen concentration as inthe case of Example 1 in advance is introduced into catalyst tower 12,and contacted with catalyst layer 13A having a relatively low power ofoxidizing NH₃ to decompose a part of the NH₃ into nitrogen and water. Atthis time, about 10% of the NH₃ in the introduced gas remains unreacted.The NH₃ remained unreacted is then contacted with catalyst 13B having arelatively high power of oxidizing NH₃ to decompose almost all remainingNH₃ into nitrogen and water, and the gas thus resulted is dischargedinto the atmosphere through pipe 14. Further, device 60 for mixing a gasis provided between catalyst layer 13A and catalyst layer B, whennecessary, so that the gas once-subjected to the decomposition withcatalyst 13A is supplied to catalyst layer 13B after the gas was madeuniform. The reaction temperature in catalyst layers 13 at this time is250 to 500° C., preferably 350 to 450° C.

EXAMPLE 3

A test for treating an effluent was conducted in the same manner as inExample 2 while using the apparatus comprising a device system similarto that shown in FIG. 4 with the exception that two catalyst towers ofcatalyst tower 50 for oxidizing NH₃ and catalyst tower 42 for reducingNOx were installed. FIG. 6 shows the device system used in this example.

In the apparatus shown in FIG. 6, air D was added to the exhaust gasdischarged from stripping tower 7 and containing ammonia at a highconcentration. The mixed gas is preheated with pre-heater 19 up to aprescribed temperature, when necessary, and then introduced intooxidizing catalyst tower 50 in which an oxidizing catalyst as a secondcomponent is packed. However, a part of the (preheated) mixed gas issupplied through by-pass line 41 to reducing catalyst tower 42 in whicha reducing catalyst as a first component is packed, and NOx isreductively decomposed into NO and N₂. The amount of the gas to besupplied to reducing catalyst tower 42 through by-pass line 41 isadjusted by controlling the opening of valve 44 based on the indicatedvalue of ammonia meter 43 placed near the outlet of reducing catalysttower 42 so that the concentration of the NH₃ in the treated gas becomesa prescribed value (for example, a value within the range of 50 to 100ppm). On oxidizing catalyst 45 in oxidizing catalyst tower 50, NH₃ isdecomposed according the equation (3) described below. However,oxidizing reactions of the following equation (1) and equation (4) bothdescribed below occur at the same time to form NO and N₂O. Then, inorder to remove the NO formed, these gases are introduced together withNH₃-containing gas supplied through by-pass line 41 into reducingcatalyst tower 42, and NO is reduced to disappear on reducing catalyst46 according the following equation (2).NH₃+5/4O₂→NO+3/2H₂O  (1)NH₃+NO+1/4O₂→N₂+3/2H₂O  (2)NH₃+3/4O₂→1/2N₂+3/2H₂O  (3)NH₃+2O₂→1/2N₂O+3/2H₂O  (4)

The gas discharged from the outlet of reducing catalyst tower 42 isreturned to stripping tower 7 by fan 25 as a part of a carrier gas afterthe temperature of the gas was raised with pre-heater 40, whennecessary.

Even in this example, it is possible to adjust the concentration of NH₃and lower the concentration of NO in the gas at the outlet of reducingcatalyst tower 42 by adjusting the concentration of the NH₃ in the gasat the outlet of tower 42 with valve 44 based on the indicated value ofammonia meter 43 placed near the outlet of reducing catalyst tower 42.Also, the amount of the energy necessary for heating a gas and liquidcan be decreased by circulating the gas discharged from the outlet ofreducing catalyst tower 42 to stripping tower 7.

In this connection, it is preferable to use an NH₃ decomposing catalyst13 as shown in FIG. 4 having a denitrating function since two catalysttowers are required and it is necessary to adjust the amount of NH₃necessary for reducing NO, by the opening of valve 44 based on theindicated value of ammonia meter 43 placed near the outlet of reducingcatalyst tower 42 in this example.

Next, a specific example in which the catalyst device as shown in FIG. 9was used is described.

EXAMPLE 4

In the device as shown in FIG. 9, a catalyst which was prepared by thesame method for preparing a catalyst as that used in Example 1 with theexception that 20 kg of a first component of catalyst and 202 g of asecond component of catalyst were used and thus the ratio of the secondcomponent to the first component (the second component/the firstcomponent) was changed to 1/99 (in this case, Pt content corresponds to1 ppm, excepting the weight of a catalyst substrate and inorganicfibers) was used as NH₃ decomposing catalyst 13A having a denitratingfunction. Likewise, NH₃ decomposing catalyst 13B having a Pt content of5 ppm and comprising 20 kg of a first component and 404 g of a secondcomponent mixed therein was prepared by the same method for preparing acatalyst as that described in Example 1. A test for treating an effluentwas conducted by using catalyst tower 12 as shown in FIG. 9 under theconditions shown in Table 3. The relation between the concentration ofthe sum of NOx and N₂O, and the reaction temperature at the time ofstarting the test is shown by curve (a) in FIG. 10.

TABLE 3 Item Condition Rate of treating effluent 1.6 L/h Amount of NH₄ ⁺in effluent 2,000 mg/L Gas flow rate at inlet of 1.3 m³/h catalyst layerGas composition NH₃: 10,000 ppm H₂O: 30% Air: the remainder Temperature350° C. Areal velocity 10 m/h

COMPARATIVE EXAMPLE 2

A test for treating effluent was conducted under the same conditions asthose in Example 4 with the exception that only catalyst 13B was used ascatalyst. The result thus obtained is shown by curve (b) in FIG. 10.

From FIG. 10, it can be understood that the concentration of the sum ofNOx and N₂O can largely be reduced in the test of Example 4 comparedwith the test in Comparative Example 2.

EXAMPLE 5

A test for treating an effluent was conducted by using the same catalystunder the same conditions as those used in Example 4 with the exceptionthat a mordenite having iron supported thereon was used in place ofcatalyst 13B. The relation between the concentration of the sum of NOxand N₂O, and the reaction temperature when a test for treating aneffluent was conducted by using the catalyst of this example under thesame conditions as in Example 4 is shown by curve (c) in FIG. 10. Aswill be understood from FIG. 10, the concentration of the sum of NOx andN₂O can further be reduced according to this example than theconcentration obtained in Example 4 since a mordenite having ironsupported thereon has a function of decomposing N₂O.

Whereas catalyst layer 13A and catalyst 13B each having a differentcomposition are arranged in series in the embodiment shown in FIG. 9,plural catalyst layers can be arranged in parallel in a catalyst deviceas shown in FIG. 11. Thus, it is possible to increase NH₃ decompositionratio to a high level and suppress the NOx concentration and N₂Oconcentration in the gas at the outlet of a catalyst tower to a lowlevel by alternately arranging plate-like catalysts comprising catalystA or catalyst C (a catalyst having a function of decomposing N₂O) in acatalyst device as shown in FIG. 11. Further, in the case where thelength of a catalyst layer cannot be extended by constraints from anapparatus, a method in which a device as shown in FIG. 11 is used iseffective. It is also effective to dispose catalysts of another shapesuch as a honeycomb shape in a catalyst device as shown in FIG. 11.

According to the embodiments as shown in FIG. 9 and FIG. 11, the problemthat when the NH₃ concentration in the gas once-treated in a catalysttower was reduced to lower than a prescribed value, NOx concentrationand N₂O concentration in the gas at the outlet of a catalyst towerbecome slightly high is resolved, and the amounts of hazardoussubstances produced can considerably be reduced.

INDUSTRIAL APPLICABILITY

The present invention can be applied to the treatment of effluentscontaining an ammonia nitrogen at a high concentration such as aneffluent discharged from a thermal power plant, and the ammonia can beremoved from the effluent at a high efficiency while reducing the amountof N₂O or the like formed at that time.

1. A method for treating an ammonia-containing effluent comprising astripping step in which the ammonia (NH₃) contained in theNH₃-containing effluent is transferred with a carrier gas from theeffluent into a gas phase, a step for adding an oxygen-containing gas tothe NH₃-containing gas produced at the stripping step, and an NH₃decomposing step in which the oxygen-containing gas and theNH₃-containing gas are contacted with a catalyst used for decomposingNH₃, at a temperature in the range of 250° C. to 450°, which catalystcomprises, as a first component, an oxide of titanium (Ti) and an oxideof one or more elements selected from the group consisting of tungsten(W), vanadium (V), and molybdenum (Mo), and, as a second component, asilica, zeolite, and/or alumina having one or more noble metals selectedfrom the group consisting of platinum (Pt), iridium (Ir), rhodium (Rh),and palladium (Pd) supported thereon, the concentration of oxygen in thegas mixture introduced into the NH₃ decomposing step being controlledwithin a range of 2 to 15% in the NH₃ decomposition step such that theN₂O concentration is reduced while the NH₃ decomposition ratio remainsgenerally unchanged.
 2. The method according to claim 1, wherein the N₂Oconcentration in the gas resulted in the NH₃ decomposing step is fromabout 4 to about 7 ppm.
 3. The method according to claim 1, wherein theoxygen concentration introduced into the NH₃ decomposing step iscontrolled within a range of 2 to 10%.
 4. The method according to claim1, wherein the oxygen concentration introduced into the NH₃ decomposingstep is controlled within a range of 5 to 10%.